EP2162407A2

EP2162407A2 - Additive for reducing hexavalent chromium, method for obtaining same and applications of said additive particularly in cement-based products

Info

Publication number: EP2162407A2
Application number: EP08805983A
Authority: EP
Inventors: Gérard Lang
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-06-12
Filing date: 2008-06-12
Publication date: 2010-03-17
Also published as: WO2009004219A3; WO2009004219A2; FR2917401A1; FR2917401B1

Abstract

The invention relates to an additive for reducing hexavalent chromium in products such as, in particular, cement, slag or gypsum, that comprises a stabilised aqueous solution of a chromium reducing agent, containing: water; a chromium reducing agent such as heptahydrated ferrous sulphate; an acid; an oxygen reducing agent dissolved in excess in water. The solution may have a pH from 4 to 6. In order to improve the grinding, the additive may further include ethanolamine. The additive can be sprayed on cement during grinding, or added in a concrete mixer.

Description

Hexavalent chromium reducing additive, process for obtaining same, and applications of this additive, especially in cementitious products.

The present invention relates to an additive for reducing hexavalent chromium contained in various products, to a process for obtaining it, and to the applications of this additive to reduce the content of hexavalent chromium, in particular in products based on cements, or other products containing chromium. hexavalent, such as during the manufacture of chromium III fluoride used for surface treatments of metals.

For decades, the health problems posed by the presence of hexavalent chromium in various products, such as mainly cements used in construction, have been known for a long time. More recently, a European directive 2003/53 / EC, concerning the reduction of hexavalent chromium Cr ⁺⁶ in cements, obliges manufacturers since January 2005 to reduce the Cr ⁺⁶ content to less than 2 ppm. It is known that Cr ⁺⁶ is recognized as responsible for serious occupational diseases affecting in particular the skin, and the above regulation intervenes in order to protect the users.

Cr ⁺⁶ in cements comes mainly from raw materials used in its manufacture, and from the wear of production equipment, such as crushers, which include chrome-containing steel parts.

The content of Cr ⁺⁶ in commonly manufactured cements is generally less than 50 ppm but well above the 2ppm value now set by the regulations. It is already known to reduce the aggressive Cr ^{+ 6} to Cr ^{+ 3} inert, by the use of strong reducing agents, which however must not modify the other qualities of the cement. Cement manufacturers essentially use two reduction processes:

addition of ferrous sulphate heptahydrate, FeSO ₄ - 7H ₂ O, in the form of crystals,

addition of stannous sulphate SnSO ₄ in suspension and aqueous solution.

The addition of crystallized FeSO ₄ , 7H ₂ O is carried out in the cement mill, at a rate of 50 kg for about 150000 kg of cement, ie only about 333 ppm of FeSO ₄ , 7H ₂ O in the cement. This amount, close to the stoichiometry corresponding to the reduction reaction Cr ⁺⁶ + 3 Fe ⁺² - * Cr ⁺³ + 3 Fe ⁺³ makes it possible to lower the Cr ⁺⁶ content from 20 ppm to less than 2 ppm.

But the addition in cement of a quantity close to the amount indicated above inevitably leads to an incomplete reaction because of the heterogeneity of the medium. Indeed, it will be readily understood that a homogeneous mixture of 50 kg of FeSO ₄ , 7H ₂ O crystals in 150000 kg of cement is impossible within a reasonable lapse of time. Areas rich in FeSO ₄ , 7H ₂ O are inevitably formed where Cr ^{+ 6} is completely reduced and residual Fe ²⁺ remains in sulphate form, as well as poor areas where the reduction is incomplete or even non-existent. In addition, in said zones rich in ferrous sulphate, the excess of Fe ²⁺ is oxidized to Fe ³⁺ and then becomes ineffective so that it can be expected that it still plays its role by prolonging the mixing time. Indeed, the oxidation of Fe ²⁺ is all the faster as the temperature is high and that there are oxidants present; or the operating conditions of the mill in which the mixing takes place are particularly suitable for this oxidation, because of the high temperature of the order of 80 to 120 ⁰ C and the presence of oxygen. An increase in the duration of A mixture aimed at improving the homogeneity of the reduction would therefore prove to be inefficient and economically detrimental by the time lost.

Furthermore, it is practically unimaginable to use ferrous sulphate in aqueous solution, because such a solution would be very unstable, because of the very rapid oxidation of Fe ²⁺ ions in water which normally contains oxygen. dissolved.

In order to overcome these disadvantages, cement manufacturers are therefore generally required to increase the amount of crystallized FeSO ₄ , 7H ₂ O by overdosing its addition by 1.5 to 2 times the expected stoichiometric amount. In addition to an increase in cost, this overdose can lead to known drawbacks, such as the clogging of the screens with excess hydrated ferrous sulfate.

Also known from FR2865727, a solution envisaged to try to obtain a more homogeneous distribution of ferrous sulfate in cement, by preparing a masterbatch in which pulverulent silica is added to the ferrous sulfate heptahydrate. The added silica is intended to increase the overall volume of the additive comprising this masterbatch so as to fluidize said ferrous sulphate, this additive then being added to the cement being milled. The pulverulent silica is thus likely to avoid or limit collages and agglomerations, and therefore to improve the homogeneity of the crystallized ferrous sulfate distribution in the cement mill, and consequently to limit the reoxidation phenomena too fast. One of the disadvantages of this method is in particular the high cost of pulverulent silica.

The use of a solution-suspension of stannous sulphate, indicated previously as an alternative method used, makes it possible, because of its liquid physical appearance, a more regular and homogeneous introduction into the cement, favorable for the reduction of chromium in the whole volume treated. However, this technique is much more expensive because of the high price of stannous sulphate, and requires the users to have an increased monitoring of the stability of the solution-suspension which is likely to decant in the storage containers before use, and thus to introduce heterogeneities of treatment into the cement. Another problem is that the action of the reducing agents is limited in time, by reoxidation of the chromium during the handling of the cement, its storage and its transport. Although this limitation in time is less when using stannous sulphate than ferrous sulphate, it does exist. This entails in particular the obligation to indicate the date of packing on the packaging of the bagged cement, the use to be done within three months from this date. The respect of a use-by date is even more difficult to ensure, and uncertain, in the case of loose cement, and the problem posed by the chrome reappears when using the cement for the manufacture of concretes or mortars . EP1533287 also describes the known conventional disadvantages encountered in the use of ferrous or stannous sulphates for the reduction of Cr ^{+ 6} and proposes, in an attempt to remedy this, to use these reducing agents in the form of an aqueous dispersion in water added with a viscosity increasing agent, to allow a more uniform dispersion of the sulfates in the solution thus made highly viscous. This viscosity also makes it possible, according to this document, to increase the quantity of sulphate per unit of liquid volume, by making it possible to keep stannous sulphate in dispersion in quantities exceeding beyond the normal solubility of sulphate. It is also indicated that the preservation of this uniform dispersion can be preserved during storage times of 28 days. The solution proposed in this document seems conceivable for the use of stannous sulphates, but is not in fact suitable for the ferrous sulphate heptahydrate.

The present invention aims to solve the problems mentioned above, and aims to provide an improved product and process for the reduction of hexavalent chromium, especially in cementitious products. It aims in particular to improve the homogeneity of the distribution of chromium reducing additives in cements during manufacture thereof. It is particularly aimed at allowing for this purpose the use of ferrous sulphate, which is less costly than stannous sulphate, under conditions improving the capacity of ferrous sulphate to have a homogeneously distributed effect in the cement, and to greatly limit the risks. reoxidation in time. It also aims to allow treatment of cement at a lower cost, as well as to allow additional or curative treatment if necessary during the manufacture of concrete using cement.

With these objectives in mind, the subject of the invention is an additive for the reduction of hexavalent chromium in products such as in particular cement, slag or plaster, characterized in that it comprises a stabilized aqueous solution, clear and homogeneous , a chromium reducing agent comprising:

- some water

- the reducing agent of chromium, - an acid, an agent reducing oxygen dissolved in excess water, the solution having a pH of 4 to 6.

The chromium reducing agent will preferably be ferrous sulphate, anhydrous or hydrated, for example heptahydrate. Stabilized solution means that the solution is stable over time and does not re-oxidize.

The subject of the invention is also a process for obtaining a solution as defined above, this process being characterized in that the following steps are carried out: determination of a dissolved oxygen level in a volume of d water, addition of an acid in an amount sufficient to maintain the pH at a value between 4 and 6, and a dissolved oxygen reducing agent in excess of the amount required to suppress said dissolved oxygen, addition of a chromium reducing agent, maintaining the pH between 4 and 6 by additional additions of acid as needed.

In summary, the general idea underlying the invention is to obtain a product based on a chromium reducing agent, such as ferrous sulfate, in aqueous solution which is stable and the rest. The invention thus makes it possible to introduce into the cement the chromium reducing agent, in particular ferrous sulphate, in particular heptahydrate, not in crystals according to the prior art, but in stable aqueous solution. This allows the introduction of the chromium reducing agent into the cement in a homogeneous manner, allowing an immediate reaction to be obtained, in very close relations with the stoichiometric reaction, and finally reducing the

Cr ⁺⁶ in Cr ⁺³ also the most homogeneous pos sible.

The oxygen reducing agent dissolved in water allows the removal of dissolved oxygen which could cause oxidation of the ferrous sulfate heptahydrate upon introduction thereof into the solution water. By preventing the oxidation of the chromium reducing agent, it is maintained in the reduced state, thereby enabling the reduction of Cr ^{+ 6} during the introduction into the cement.

The acid added to the solution is intended to maintain it at a sufficient acidity to prevent the precipitation of hydroxides and subsequent reoxidation. The solution of the chromium reducing agent manufactured according to the invention remains stable and clear, and retains its effectiveness, for several months.

As indicated above, preferably, the chromium reducing agent is ferrous sulphate, especially ferrous sulphate heptahydrate. However, it is also possible to use, for example, manganese sulphate II, capable of improving the mechanical property of cement, or other salts such as alkaline, alkaline earth or metallic hypophosphites or phosphites.

Preferably, the additive comprises, in weight proportions, from 5 to 15%, preferably 10%, of iron sulphate heptahydrate, from 3 to 3.5% of acid, and a quantity of dissolved oxygen reducing agent. in excess water of 10% by weight of the amount required to completely reduce the dissolved oxygen. The excess of this reducing agent of the order of 10% by weight makes it possible to remain permanently in a reducing medium and to prevent reoxygenation of the water during the manufacturing process. The acid may be chosen from, in particular, formic acid, sulfuric acid, hydrochloric acid, acetic acid, glyoxilic acid, or a mixture of two or more of these. Preferably, use formic acid which has the advantage of having a reducing power.

The reducing agent for dissolved oxygen in water may be chosen from hydrazine or a salt thereof, acid or neutral alkaline or alkaline earth sulphites, such as sodium sulphite, nitrites, hyposulphites, thiosulfates, hydroxylamine ascorbic acid, or a mixture of two or more thereof. Preferably, hydrazine hydrate will be used which has the advantage of being an immediate reducer of oxygen dissolved in water, at low concentration.

The process according to the invention aims, as indicated previously, to allow the production of a stable solution of iron sulphate heptahydrate. Typically, for 500 liters of water, about 16 kg of acid, 50 kg of crystals of iron sulfate heptahydrate, or equivalent mass ratios are introduced.

The subject of the invention is also a combined additive for the reduction of hexavalent chromium in cement and the improvement of grinding, this combined additive comprising in aqueous solution

a chromium reducing agent, such as iron sulphate heptahydrate,

an acid, in an amount sufficient to maintain the solution at a pH of 4 to 6, an oxygen reducing agent dissolved in water in an amount sufficient to reduce all of the dissolved oxygen in the water, an ethanolamine , preferentially from the triethanolamine.

A process for obtaining this combined additive is characterized in that the solution of the chromium reducing agent obtained as indicated above is mixed with a solution obtained by the following steps: determination of a dissolved oxygen level in a volume of water,

adding an acid in an amount sufficient to maintain the pH at a value between 4 and 6, and a dissolved oxygen reducing agent in excess of the amount required to suppress said dissolved oxygen,

- addition of ethanolamine, maintaining the pH between 4 and 6 by additional additions of acid as needed.

This combined additive actually makes it possible to propose a new product which, in the case of the use of iron sulphate heptahydrate, combines the iron sulphate used to reduce the Cr ⁺⁶ content in the cement and the triethanolamine which is used. as the grinding aid, while the two products iron sulfate heptahydrate and ethanolamine are normally incompatible with each other in aqueous solution, since the strongly basic ethanolamine normally causes the precipitation of ferrous hydroxide and its rapid conversion to ferric hydroxide by oxidation due to the presence of dissolved oxygen in the water of preparation, according to the reactions:

Fe ²⁺ + H ₂ O -> Fe (OH) ₂ Fe (OH) ₂ + O ₂ -> Fe (OH) ₃

The prior elimination of oxygen dissolved in water by the reducing agent such as hydrazine makes it possible to avoid the oxidation of iron sulfate heptahydrate thereafter. The excess of hydrazine ensures that the oxygen that can be brought into the water during the process, for example by the mixing carried out to obtain the mixture, will also be effectively reduced and eliminated. The maintenance in acid solution during the addition of ethanolamine and subsequently avoids the harmful effects thereof recounted above. Overall, it is thus possible to obtain a stable solution of Fe ²⁺ and of ethanolamine, which can be preserved and then used by an addition in the cement mill, ensuring both the reduction of chromium and the functions of grinding aid. ethanolamine.

The combined additive can also be obtained by adding ethanolamine while maintaining the pH between 4 and 6 by additional additions of acid as needed before or after the addition of the chromium reducing agent.

According to a particular provision, the combined additive comprises, in weight proportions, from 4 to 6% of iron sulphate heptahydrate, from 4 to 6% of triethanolamine, from 3 to 3.5% of acid, and a quantity of Oxygen reducing agent dissolved in the excess water of 10% by weight of the amount required to completely reduce the dissolved oxygen in the water.

The invention also relates to a method of using the previously defined combined additive for reducing hexavalent chromium in cements and improving grinding, characterized in that the additive is sprayed on the cement in progress grinding in the mill in a proportion of 900 to 1100 liters of additive for 150000 kg of cement.

The invention also relates to a method of using the additive containing the chromium reducing agent for the reduction of hexavalent chromium in cements during the manufacture of concrete, characterized in that the additive is added in the concrete mixer. This In particular, this method can be used to ensure the reduction of Cr ^{+ 6} , and / or to reinforce the absence of Cr ^{+ 6} , in concretes made from old cements, or of uncertain provenance and age.

According to a particular embodiment of the invention, a solution A of ferrous sulphate heptahydrate is prepared as follows:

In a mixing tank, 500 liters of water are introduced. The determination of the dissolved oxygen in this water is carried out, or the dissolved oxygen content resulting from the solubility tables according to the French standard NFT90-032 of April 1975, which provides the dissolved oxygen content in the water depending in particular on its temperature and its salinity.

16 kg of 99% formic acid or other acid such as sulfuric acid are then introduced with stirring. After homogenization, slowly stirring is introduced a calculated amount of hydrazine, or its salts, or an acidic acid-alkaline or alkaline earth alkaline. The known reactions of destruction of dissolved oxygen are as follows:

H ₂ N-NH ₂ + O ₂ -> N ₂ + 2H ₂ O

2 Na ₂ SO ₃ + O ₂ -> 2 Na ₂ SO ₄ An excess of approximately 10% of hydrazine or of the previously used reducing agent is added in order to remain permanently in a reducing medium.

Then 50 g of FeSO ₄ , 7H ₂ O are introduced into the vessel, still under slow stirring in order to avoid the reintroduction of oxygen in the water, and this stirring is maintained until the FeSO ₄ completely dissolves, 7H ₂ O and obtaining a clear solution. During this dissolution, it is ensured that the pH remains well below pH 5, and is corrected if necessary by adding formic or sulfuric acid.

The manufacture of solution A is now complete and it can be stored for several months in the absence of air, or use as such including direct addition in the concrete mixer as indicated above.

In order to obtain the combined additive, a solution B of ethanolamine, preferably triethanolamine, is also prepared according to the following method: In a mixing tank, 500 liters of water are introduced and, as for the Preparation of solution A is followed by addition of hydrazine, sulfite or the like, to remove dissolved oxygen in water, and to obtain an excess of hydrazine of 10% by weight. 16 kg of 99% formic acid are then introduced with slow stirring. After homogenization, 50 kg of 99% triethanolamine are introduced. Throughout this addition, the pH is checked and maintained below pH5 by adding formic or sulfuric acid in an adequate amount. The solution B obtained is clear and its pH is adjusted to pH 5 plus or minus 1, with formic acid or triethanolamine.

The two solutions A and B are then mixed, introducing indifferently A into B or B into A, with slow unaerated stirring, and keeping the pH at about 5. The resulting solution, constituting the combined additive, has the following composition:

FeSO ₄ , 7H ₂ O 50 kg

99% triethanolamine 50 kg 99% formic acid 32 kg

Dissolved oxygen reducer, in sufficient quantity,

QSP water 1000 liters

The solution obtained remains clear and stable for several months in a closed container, and with a pH of 4-6.

It is already known to use triethanolamine as clinker grinding aid, to use its lubricating properties to facilitate the grinding of cement. The addition of triethanolamine in the solution makes it possible to combine the beneficial effect it brings to the reduction of chromium, with its already known role of grinding aid.

The introduction of this combined additive into the cement mill by spraying greatly increases the efficiency of reducing Cr ⁺⁶ to Cr ⁺³ . In fact, there is a more homogeneous mixture with the cement, and thus eliminates the disparity between rich zones and low-reducer zones, which exists in the process of using ferrous sulphate crystals.

In addition, two active products are introduced into the mill in a single injection, which enables gains to be made during the manufacture of the cement, and also significant gains in operating costs, the screens remaining clean.

The combined additive is sprayed at a rate of 1000 liters per 150 000 kg of cement. This addition makes it possible to lower a content of 20 ppm of Cr ⁺⁶ at a content of less than 2 ppm. It should be noted incidentally that hydrazine, which has also been proposed as a reducing agent to directly reduce Cr ^{+ 6,} may have some toxicity at the high concentrations then required, of the order of 200 to 700 ppm per tonne of cement, but not at all. useful contents according to the invention, which are only of the order of 1 ppm.

According to the invention, the hydrazine serves to eliminate dissolved oxygen, and to avoid a possible reappearance of dissolved oxygen during the manufacturing process or the use of the additive. The invention makes it possible in particular to limit the hydrazine content by adding the acid which avoids the oxidation of ferrous sulphate. This acid also makes it possible to compensate for the basicity of the hydrazine and thus to keep the solution sufficiently acidic to avoid reoxidation of the ferrous sulphate. Although other chromium reducers may be used, such as calcium sulphite, etc., iron sulphate heptahydrate is particularly preferred because of its low cost.

Solutions A and B can be obtained substantially more concentrated than as indicated, for example up to three times more concentrated.

As already indicated, it is also possible to use other acids than formic acid, for example sulfuric acid, hydrochloric acid, acetic acid, glyoxilic acid, or a mixture of two or more thereof. this. It is also possible to use other oxygen reducers than hydrazine, for example alkaline or alkaline earth acid or neutral sulphites, such as sodium sulphite, nitrites, hyposulphites, thiosulfates, hydroxylamine, ascorbic acid, or a mixture of two or more of these.

As an indication, variants of the additive could be:

Water and hydrazine 1000 1

Sulphite calcium acid 20kg Ferrous sulphate heptahydrate 30kg

Or :

Water and hydrazine 1000 1

Calcium sulphite 25kg

Calcium sulphite 10 kg

This last variant is advantageous because it causes the formation of calcium sulphate, or plaster, without coloring the cement by the presence of sulphate ferric or ferric hydroxide as in the previous embodiments.

Claims

1. Process for obtaining a solution for the reduction of hexavalent chromium in products such as in particular cement, slag or plaster, characterized in that the following steps are carried out: determination of a rate of dissolved oxygen in a volume of water,

- Adding a chromium reducing agent, maintaining the pH between 4 and 6 by additional additions of acid as needed.

2. Method according to claim 1, characterized in that the chromium reducing agent is ferrous sulphate, anhydrous or hydrated, especially heptahydrate.

3. Method according to claim 2, characterized in that the solution contains in final, in weight proportions, from 5 to 15%, preferably 10%, of iron sulfate heptahydrate, from 3 to 3.5% of acid, and a quantity of oxygen reducing agent dissolved in the excess water of 10% by weight of the amount required to completely reduce the dissolved oxygen.

4. Method according to one of claims 1 to 3, characterized in that the acid is selected from formic acid, sulfuric acid, hydrochloric acid, acetic acid, glyoxilic acid, or a mixture of two or more of these.

5. Method according to one of claims 1 to 4, characterized in that the oxygen reducing agent dissolved in water is chosen from hydrazine or a salt thereof, acid or neutral alkaline or alkaline earth sulphites, such as sodium sulphite, nitrites, hyposulphites, thiosulfates, hydroxylamine, ascorbic acid, or a mixture of two or more of them.

6. Method according to one of claims 2 to 5, characterized in that, for 500 liters of water, is introduced about 16 kg of acid, 50kg of iron sulfate crystals heptahydrate, or equivalent mass ratios.

7. Method according to one of claims 1 to 5, characterized in that, for the purpose of improving the grinding, the solution obtained is mixed with a solution obtained by the following steps: determination of a dissolved oxygen level in a volume of water,

8. Method according to one of claims 1 to 5, characterized in that, in order to improve the grinding, ethanolamine is added to the solution obtained by maintaining the pH between 4 and 6 by complementary additions of acid as needed, before or after the addition of the chromium reducing agent.

9. Additive for the reduction of hexavalent chromium in products such as in particular cement, slag or plaster, comprising a stabilized aqueous solution of a chromium reducing agent comprising: - some water

- the reducing agent of chromium,

an acid, an oxygen reducing agent dissolved in excess water, the solution having a pH of 4 to 6 and being characterized in that it is obtained by the method according to one of claims 1 to 5; .

10. Additive according to claim 9, characterized in that the chromium reducing agent is ferrous sulphate, anhydrous or hydrated, especially heptahydrate.

11. Additive according to claim 9, characterized in that, in order to improve grinding, it further comprises ethanolamine.

12. Additive according to claim 11, characterized in that it comprises, in weight proportions, from 4 to 6% of iron sulfate heptahydrate, from 4 to 6% of triethanolamine, from 3 to 3.5% of acid, and an amount of oxygen reducing agent dissolved in the excess water of 10% by weight of the amount required to completely reduce the dissolved oxygen in the water.

13. Use of the additive according to claim 11 or 12 for the reduction of hexavalent chromium in cements and the improvement of grinding, characterized in that the additive is sprayed on the cement being milled in the grinder. a proportion of 900 to 1100 liters of additive for 150000 kg of cement.

14. Use of the additive according to claim 9 or 10 for the reduction of hexavalent chromium in cements during the manufacture of concrete, characterized in that the additive is added to the concrete mixer.